1. Field of the Invention
The present invention generally relates to a system for correcting distortion of an image displayed on a display device, and more particularly to a method for correcting distortion of an image displayed on a screen of a display unit so that the distortion is canceled, a distortion detecting unit for detecting such distortion of an image, a distortion correcting unit for correcting distortion of an image displayed on a screen of a display unit based on the detected distortion of the image and a display apparatus having such a distortion correcting unit.
2. Description of the Related Art
As shown in FIG. 1, in a display device 10 using a CRT (Cathode-Ray Tube), when a rectangle S is displayed on a screen 10a without correction of distortion of a displayed image, the rectangle S is distorted so that four corners of the rectangle S are expanded. This phenomenon is caused, as shown in FIG. 2, by the difference between the curvature center of a curved tube surface (corresponding to the display screen 10a) of the CRT 11 and the deflection center of an electron-beam emitted from an electron gun.
There are basic patterns of the distortion of the rectangle image as shown in FIGS. 3A, 3B, 3C and 3D. The distortion shown in FIG. 3A is often called a pin-cushion distortion. In the pin-cushion distortion, lines facing each other are distorted so as to be curved in opposite directions (see FIG. 1). The distortion shown in Fig. 3B is often called a bowed distortion. In the bowed distortion, lines facing each other are distorted so as to be curved in the same direction. The distortion shown in FIG. 3C is often called a trapezoidal distortion. In the trapezoidal distortion, lines facing each other are distorted so as to be inclined in opposite directions. The distortion shown in FIG. 3D is often called a rhomboidal distortion. In the rhomboidal distortion, lines facing each other are distorted so as to be inclined in the same direction. Although, in FIGS. 3A, 3B, 3C and 3D, the right and left lines of the rectangle are distorted, the upper and lower lines of the rectangle have the same distortion patterns.
In addition, as shown in FIG. 4, lines L1, L2, L3, L4, L5, L6, L7, L8 and L9 are displayed on the screen 10a of the display device 10 without correction of distortion of displayed images. These lines should be arranged at constant intervals in a direction parallel to the horizontal direction. However, due to the lack of correction of distortion of the displayed images, as the lines get closer to the circumference of the screen 10a, the distance between lines increases (deterioration of the vertical linearity). This is also caused by the difference between the curvature center of the tube surface of the CRT 11 and the deflection center of the electron beam.
Conventionally, to correct such distortion of images on the screen (the rectangular screen), correction current applied to a deflection unit (a deflection coil) is adjusted while a test pattern displayed on the screen is being observed. For example, in a case where the pin-cushion distortion as shown in FIG. 3A is corrected, the distorted lines are regarded as being parabolic-wave (square-wave) shaped lines. The correction current is applied to the deflection unit so that the parabolic-wave shaped lines are canceled. The distortion of the rectangle S displayed on the screen 10a as described above is caused by a structure of the CRT 11. Thus, due to adjustment of the correction current supplied to the deflection unit, the distortion may be corrected.
However, the distortion of the image displayed on the screen 10a of the display device 10 may be caused by other factors, such as variation of a mounting position of the electron gun in the CRT, variation of a mounting position of the deflection yoke, influence of a fine adjustment magnet, instability of a high-voltage source and loss of a deflection current. Due to the above factors, a line on the screen 10 may be distorted slightly but complicatedly as a line L shown in FIG. 5. Such complex distortion is not eliminated by the conventional method for adjusting the correction current supplied to the deflection unit while a display condition on the screen is being observed.
As a result, in a case where a high-quality image displayed on the screen of a high-definition display device is required, the conventional method for correcting the distortion of images, as described above, can not sufficiently fulfill the requirement.
Thus, methods of finely correcting the complex distortion of images have been proposed (Japanese Patent Laid Open Application Nos. 53-149712, 4-23692, 6-327019 and 7-264611). In the respective proposed methods, a predetermined pattern is displayed on the screen and the difference between a position of each point on the pattern actually displayed on the screen and a corresponding position of a point to be positioned on the pattern is stored as correction data. When an image is displayed, the deflection unit is controlled using the correction data.
However, in the respective methods, conventionally proposed, for correcting the distortion of the displayed image, a large amount of data is needed in order to accurately correct the distortion of the displayed image. The differences between the positions of respective points on the actually displayed image and the positions of corresponding points to be positioned on the image are stored in the correction data. In order to accurately correct distortion of the displayed image, a large amount of correction data must be stored in the display device. As a result, an extensive area for the correction data must be prepared in an internal memory of the display device.